September 15, 2017
4 min read

Thymic cyst causes false-positive radioactive iodine scan

You've successfully added to your alerts. You will receive an email when new content is published.

Click Here to Manage Email Alerts

We were unable to process your request. Please try again later. If you continue to have this issue please contact

The patient was referred because of an abnormal posttherapy scan after radioactive iodine ablation for thyroid cancer. The patient initially presented with a large multinodular goiter. Fine-needle biopsy of a left thyroid nodule was consistent with papillary thyroid carcinoma that was Bethesda VI, and the right nodule was suspicious for papillary thyroid carcinoma that was Bethesda V.

Stephanie L. Lee
Matthew P. Gilbert

A total thyroidectomy revealed a 3.6 cm x 2.6 cm x 1.4 cm poorly differentiated thyroid carcinoma in the left lobe and isthmus with extensive invasion of the tumor capsule and lymphovascular space with negative surgical margins. BRAF V600E mutation analysis was negative. In addition, there was a micropapillary thyroid carcinoma 0.4 cm in the right lobe.

Imaging, diagnosis

The patient was withdrawn from thyroid hormone and allowed to become hypothyroid. Planar imaging on diagnostic radioactive iodine scan showed bilateral thyroid remnants, a left lateral level 2/3 metastatic node and faint radiotracer persistence in the anterior mediastinum interpreted as uptake in the esophagus at the level of the aortic arch (Figure 1). After treatment with 150.9 mCi iodine-131, the posttherapy scan showed a “new” area of radiotracer in the anterior mediastinum (Figure 2), which may, in fact, be the same area of trapping seen on the diagnostic scan. Comparison of the two planar scans could not determine the location of the isotope trapping. The second set of images obtained after iodine-131 therapy was performed with single-photon emission CT, which allowed fusion of tomographic images and a low-dose attenuation correction (nondiagnostic) CT scan. SPECT imaging identified a rounded mass located just posterior to the upper sternum at the level of the aortic arch on the nondiagnostic CT scan.

Figure 1. Radioactive iodine whole body scan. (A,B) Hypothyroid diagnostic scan. (C) Hypothyroid posttherapy scan. The diagnostic pre-therapy whole body scan was performed on an older gamma camera that imaged the head and neck (A) separately from the chest and abdomen (B). Radiotracer was localized on planar images to bilateral thyroid remnants (yellow arrow), a metastatic node in left level 2/3 and physiological localization in the nasopharynx, salivary glands (green arrow) and gastrointestinal tract (orange arrow). Faint uptake was seen on the diagnostic pre-therapy scans (A,B) that was located at the division point of the neck and chest images (black arrows). After 150 mCi iodine-131, the posttherapy scan (C) was performed on a newer gamma camera with SPECT imaging showing the intense uptake (red arrow) was located in a low-density mass in the anterior mediastinum. Source: Stephanie L. Lee, MD, PhD, ECNU. Reprinted with permission.

Because the patient had the radioactive iodine therapy 1 week before, and to avoid iodine-containing CT contrast, an MRI with gadolinium was obtained that showed a 3.7 cm x 2.2 cm x 1 cm (cranial caudal x TR X AP) mass in the anterior mediastinum. The mass was cystic based on a hyperintense appearance on a T2-weighted inversion recovery single-shot turbo spin-echo sequence. The mass did not show enhancement after gadolinium contrast and no diffusion restriction.

The differential diagnosis after imaging included a cystic metastatic node or a benign developmental cyst. CT and MRI cannot always reliably differentiate between solid and cystic masses in the anterior mediastinum. The patient was scheduled for resection of the mass 3 months after the radioactive iodine therapy. A preoperative CT scan showed a nonspecific cystic lesion with a low Hounsfield unit of 17 in the anterior mediastinum. The mass measured up to 3.7 cm with no contrast enhancement and a differential diagnosis of a cystic metastatic node, a pericardial cyst, bronchogenic cyst or thymic cyst.

Figure 2. MRI and CT imaging. CT imaging (A-D). MRI imaging (E, F). A low-density mass (HU: 17; red arrows) with well-defined smooth margins that did not enhance after contrast enhancement was found immediately behind the upper sternum on axial (A), coronal (B) and sagittal (C) views. The mass is located within the typical sail-shaped thymus (yellow arrow) posterior to the sternum in the anterior mediastinum on an axial CT image. The mass is shown to be cystic based on the hyperintensity (red arrow on E and F) on T2-weighted inversion recovery single-shot turbo spin-echo (SSTSE) sequence of an MRI scan.

Distinguishing among cysts

Because the diagnosis was not clear, the patient had surgery to remove the cystic structure. Surgical resection showed a benign thymic cyst lined by cuboidal cells surrounded by involuted thymus tissue. No metastatic thyroid cancer was found in the surgical specimen. His initial staging was American Joint Committee on Cancer stage IVB with an American Thyroid Association intermediate risk of recurrence. After 3 years, the patient has had an excellent response to therapy with a thyroglobulin level less than 0.1 ng/mL and negative thyroglobulin antibody. His current ATA risk is low for recurrence.

Cystic masses of the mediastinum contain fluid and are lined with epithelium. Differential diagnosis includes congenital benign cysts (ie, bronchogenic, esophageal duplication, neurenteric, pericardial and thymic cysts), meningocele, mature cystic teratoma and lymphangioma. Many mediastinal tumors may undergo cystic degeneration, including metastatic adenopathy, thymomas, Hodgkin disease, germ cell tumors and mediastinal carcinomas. These tumors with extensive cystic degeneration may be difficult to distinguish from benign cysts on both CT and MRI.

Simple congenital thymic cysts usually appear low density on CT. However, some thymic cysts may have increased CT density after hemorrhage or infection and may be misdiagnosed as solid masses. With MRI, thymic cysts demonstrate the typical characteristics of fluid (ie, low signal intensity on T1-weighted images, uniform high signal intensity on T2-weighted images), but after hemorrhage or infection, the cysts can have high signal intensity on both T1- and T2-weighted images. Conventional CT or MRI findings cannot always provide a confident diagnosis of a thymic cyst and sometimes require surgical excision in the patient with an initial immediate-high risk for metastatic disease.


Disclosures: Gilbert and Lee report no relevant financial disclosures.